1. Field of the Invention
The present invention relates to resonators of the type having an AT type cut quartz plate and a pair of main electrodes applied to the plate to create an electric field, and more particularly to an improvement thereon for controlling undesirable resonance frequencies thereon.
2. Description of Prior Art
The wide use of quartz crystal resonators in the electronic field is well known, especially in filters and in oscillators. Conventional prior art resonators are generally composed of a quartz plate and a pair of electrodes applied on each surface of the plate. FIGS. 1-3 herein illustrate schematic representations of the well known prior art and shall briefly be described hereinafter in order to clarify ideas and to better outline the aspects of the technical field to which the present invention herein pertains.
In FIG. 1, a quartz crystal C is shown, defined by the three orthogonal axes X.sub.1, X.sub.2, X.sub.3 in the conventional trigonal system of crystal architecture employed for the quartz crystal C. A quartz plate having an AT orientation is conventionally obtained by an AT cut from this crystal C and contains the a digonal axis (X.sub.1) and forms an angle of 35.degree.15' with the trigonal axis (X.sub.3) as shown illustratively in FIG. 1. For a better representation of the AT cut plate in FIG. 1 let us consider a plate one (L), rotated in an anti-clockwise direction about (X.sub.1) and (X.sub.2 ') and defined by the axes (X.sub.2 '), (X.sub.3 ') and X.sub.1. FIG. 2 represents a partial schematic view of a portion of this plate L with axes (X.sub.1) and (X.sub.2 ') lying in the plane of the paper. T represents the thickness of plate L.
It is well known that if a suitable electric field E is applied to the small plate L in a direction (X.sub.2 ') (for example by means of the electrodes El.sub.1 and El.sub.2) a deformation (D) with respect to the median plane (M) (FIG. 2) is produced through a piezo-electric effect. This deformation generates an acoustic wave which, starting from the excitation point, propagates with a velocity which is a function of the mass and of the elasticity of the quartz and the geometry of the plate and of the electrodes (for example El.sub.1 and El.sub.2). In general, the electrodes in question are conventionally obtained by depositing metallic layers on the surface of the plate (L).
FIG. 3 represents a perspective view of a plate (L) of a parallelepipedon shape, which has in particular an upper surface (F.sub.1) and a lower surface (F.sub.2) separated by a thickness of quartz material T. The electrode El.sub.1 consists of a metallic deposit, having rectangular boundaries 1 and 2 and a very small thickness 3 with respect to the thickness (T) of the plate. The rectangular portion of (El.sub.1) is extended by means of a lead G.sub.1 to the edge 4 of plate (L) so as to form a terminal. The electrode (El.sub.2) is not visible in FIG. 3 because it is covered by (El.sub.1). Electrode (El.sub.2) must thus be imagined in the same position as (El.sub.1) but deposited on face (F.sub.2) of (L). Only the lead (G.sub.2) of electrode (El.sub.2) is indicated with dotted lines which terminate at edge 5 forming the second terminal. Conventional resonators of the type represented in FIG. 3 have the considerable inconvenience of exciting, besides the fundamental resonance frequency, a whole series of undesired or spurious resonance frequencies, the characteristics of which depend substantially on the geometry of the system consisting of the plate (L) and of the electrodes (for example El.sub.1 and El.sub.2). As a consequence, by acting suitably on these geometries it is possible to influence the position and the merit or Q- factor of the fundamental frequency and the spurious resonance frequencies. However, it is extremely difficult in the industrial manufacturing procedures employed for such resonators, on a large scale, to maintain geometries of plates and/or of perfectly identical electrodes and, therefore, it is difficult to succeed in manufacturing resonators with characteristics that coincide perfectly. Because of this, many inconveniences arise in the devices and the circuits in which these resonators are inserted which resonators, even though having substantially identical geometries (within the normal limits of industrial reproducibility), present very different characteristics. The present invention is directed to a resolution of the control and repeatability problem of the characteristics of such quartz resonators and overcomes the disadvantages of the prior art.